Due to the development of genetic recombination techniques, the market for protein pharmaceutical products such as therapeutic proteins and antibody drugs has rapidly expanded. In particular, antibody drugs have high specificity and do not cause an adverse immunoreaction even if they are administered to the human body, and therefore, the development thereof has been actively performed.
As a host cell in which a protein pharmaceutical typified by an antibody drug is produced, a microorganism, a yeast, an insect, an animal or plant cell, a transgenic animal or plant cell, or the like can be used. In order for the protein pharmaceutical to have biological activity or immunogenicity, post-translational modification such as folding or glycosylation is essential, and therefore a microorganism with which complicated post-translational modification cannot be performed or a plant having a different glycan structure is not suitable as a host cell operating as a bioreactor. The use of a cultured mammalian cell such as a CHO cell which is from a species closely related to humans is currently standard considering that such a cell has a glycan structure similar to that of humans and is safe, and post-translational modification can be performed using such a cell.
In cases where a cultured mammalian cell is used as a host cell, there are the problems that the growth rate is low, the productivity is low, the cost is high, etc., as compared with a microorganism or the like (see Non-Patent Document 1). In addition, in order to use a protein pharmaceutical product in a clinical trial, it is necessary to administer a large amount of the product. Therefore, the lack of production ability thereof is also a worldwide problem. Accordingly, in order to improve the productivity of a foreign gene in a cultured mammalian cell, a lot of studies of promoters, enhancers, drug selection markers, gene amplification and culturing engineering techniques, and the like have been performed so far. However, the current situation is that a system capable of uniformly increasing gene expression has not yet been established. As one of the causes of the low productivity of a foreign protein, a “position effect” is considered (see Non-Patent Document 2). When a foreign gene is introduced into a host cell, it is randomly integrated into the host chromosomal genome, and the transcription of the foreign gene is greatly affected by DNA around the region where the foreign gene has been integrated. A position effect is affected by factors such as the insertion site, copy number, structure, etc. of the foreign gene, however, it is very difficult to control the insertion site in the chromosome.
In order to solve the problem, regulatory polynucleotide sequences (also known as DNA elements) such as a locus control region (LCR), a scaffold/matrix attachment region (S/MAR), an insulator, a ubiquitous chromatin opening element (UCOE), and an anti-repressor (STAR element) have recently been identified (see Non-Patent Documents 3 to 6). A LCR is not required to open the chromatin structure at an endogenous gene locus. However, a LCR is a transcription regulatory element having an ability to open the chromatin structure around the DNA where the foreign gene has been integrated and to remodel a wide range of chromatin when it is used along with a foreign gene expression unit, and is said to require an AT-rich region (see Non-Patent Document 7).
The above-mentioned DNA element typified by LCR is often used in combination with a promoter, and it is known that in cases where a DNA element is used in combination with a promoter, the expression level of a foreign gene is increased as compared with cases where only the promoter is used. However, very few types of DNA elements have been reported so far, and the various mechanisms contributing to the enhancement of foreign gene expression are different from one another. Further, even if a DNA element and a promoter are used in combination, sufficient amounts of a therapeutic protein under the control of the DNA element and the promoter are not produced. Therefore, it cannot be said that sufficient knowledge of a DNA element capable of increasing the productivity of a foreign protein has been obtained.
Accordingly, an object of the invention is to provide a method for increasing the production of a foreign protein to be used in a protein pharmaceutical product using a DNA element having high activity in enhancing foreign gene expression in a host cell such as a cultured mammalian cell.